FIG. 1 shows the top view of a prior art z-cut lithium niobate modulator 2, consisting of an optical waveguide 4 that is split into two paths 4a and 4b, which are recombined later, along with a traveling-wave set of electrodes. The two waveguides 4a and 4b form the arms of a Mach-Zehnder interferometer. The electrodes consist of a signal electrode 5a, which is positioned directly over top of one of the waveguides 4a, and two ground electrodes 5b, 5c, one of which, 5b is on top of the second waveguide 4b. In operation, the field from the electrodes creates a difference in optical phase between the two arms of the interferometer in response to an applied voltage. The constructive and destructive interference between the optical waves in the two waveguides creates intensity modulation in response to the voltage. In normal operation, a DC bias voltage is applied along with the high-speed modulation voltage. The slowly varying bias voltage is used to set the interferometer nominally to the half power point, in the absence of modulation voltage. The bias voltage tracks changes in differential optical phase occurring over time, maintaining the interferometer nominally at the half power point or quadrature point. If the changes in differential optical phase are large, perhaps due to changes in ambient temperature, then the required bias voltage Vb will become exceedingly large, causing a failure or errors in transmission of digital or analog signals. A Z-cut modulator is described in U.S. patent application Ser. No. 10/720,796 incorporated herein by reference. Other prior art patents in a similar field of endeavor are U.S. Pat. Nos. 5,790,719; 6,544,431; 6,584,240 and 6,721,085 incorporated herein by reference.
Referring now to FIG. 2, a cross-sectional view of a portion of a conventional Mach Zehnder interferometer modulator is shown, with three coplanar strip electrodes 10, 12 and 14 having only a single drive voltage. This externally modulated system has a Z-cut LiNbO3 substrate, which requires a lower drive voltage than is generally required for modulators having an X-axis or X-cut crystal orientation. The Z-cut LiNbO3 substrate has an electro-optical effect, which provides a broadband low drive voltage modulator. The electrodes are shown to be disposed over waveguides 15 and 17. The Z-cut lithium niobate modulator has no slots in the substrate. A buffer layer partially isolates the electrodes from the substrate. The high permittivity of the substrate lowers electrode impedance and microwave velocity to undesirably low values. The isolating effect of the buffer layer restores desired impedance and microwave velocity at the expense of modulation efficiency. An impedance of 40 to 50 Ohms is desirable for efficient transfer of power from the driver circuit to the modulator. In addition, the microwave velocity is typically designed to be the same as the optical velocity, in order to obtain the largest possible modulator bandwidth for the selected electrode length. The buffer layer also isolates the electrode optically from the waveguides. The electrodes would dramatically increase the loss of light in the waveguide, if they were formed directly on top of the waveguides.
Referring now to FIG. 3, a prior art modulator similar to one described by K. Noguchi, et al., “Millimeter-wave TiLiNbO3 optical modulators,” IEEE Journal of Lightwave Technology, Vol. 16, No. 4, April 1998, pp. 615–619, is shown. The surface of the substrate 30 is etched or machined everywhere except where the waveguides 32, 34 are located. The removal of the substrate creates ridges 35a, 35b containing the waveguides 32, 34 which confine the field flux from the electrodes to the waveguide region, thereby increasing the amount of modulation. These ridges 35a, 35b also increase the impedance and velocity, allowing for a thinner buffer layer, and hence, further reduction in drive voltage. Finally, the ridges 35a, 35b can narrow the optical beam width in the waveguide, resulting in some further incremental improvement in modulation efficiency. The term ridge used within this specification is to mean a long narrow elevation or striation. In accordance with this specification, a ridge has at least one valley or slot below its peak or plateau. In some instances a ridge may have a slot or valley on both sides of its peak or crest. For example three ridges R1, R2 and R3 that are disposed adjacent and parallel to one another have at least two valleys V1, V2 therebetween, as follows: R1 V1 R2 V2 R3. Notwithstanding, three ridges R1 R2 and R3 adjacent and parallel to one another may have four valleys adjacent thereto, as follows: V1 R1 V2 R2 V3 R3 V4, or may have three valleys V1 R1 V2 R2 V3 R3 or R1 V1 R2 V2 R3 V3.
The waveguide regions and ridges shown in FIG. 3 protrude from the surface of the substrate, making them more prone to damage during wafer processing. In addition, the narrow signal electrode 34a is higher than the ground electrode 34c on the left, making it more prone to damage during wafer processing. It is desirable to have the signal electrode 34a lower in height than either ground electrode 34b or 34c. In such as case, the ground electrodes provide protection for the narrow signal electrode 34a. 
FIG. 4 shows another prior art modulator developed at JDS Uniphase having slots 40a, 40b defined in the substrate to reduce the modulation drive voltage. The slots are located in the electrode gaps between the signal electrode 42 and ground electrodes 44a, 44b. The provision of slots by removal or etching of material is a simpler process than the provision of ridges by growing electro-optic material where ridges are to be provided; notwithstanding, both techniques are within the scope of the present invention. The drive voltage reduction is nearly the same as with ridges, for a given electrode impedance and microwave velocity. In FIG. 4, the signal electrode 42 is always the same height or lower than either ground 44a or 44b, making the design more robust to wafer processing. FIG. 5 shows another prior art modulator with narrow slots 51a, 51b, 51c, 51d described in U.S. Pat. No. 6,545,791, designed for reducing the modulation voltage.
It is found that the design shown in FIG. 4 is more efficient than the design of FIG. 5. In general, the deeper and wider the slots, and more the lithium niobate that is removed, the greater the voltage reduction that is possible. However, there are fabrication limits to the depth of the slots. In addition, the presence of the slots and thinner buffer increases optical loss in the waveguide. Hence, there is a trade-off between voltage improvement and increase in optical loss.
It is an object of this invention to provide modulator which has substantially low loss and has an improvement in voltage reduction.